Which electrolyte is the most crucial during phase 3 of a cardiac action potential?

During phase 3 of a cardiac action potential, the most crucial electrolyte is potassium, K+. This phase is characterized primarily by the repolarization of the cardiac myocyte, which is achieved through the outflow of K+ ions from the cell.

As action potentials progress in cardiac cells, depolarization occurs primarily due to the influx of sodium (Na+) and calcium (Ca2+) ions during phases 0 and 2, respectively. However, in phase 3, the closure of calcium channels and the opening of potassium channels allow K+ to exit the cell, leading to repolarization. This movement of K+ is essential to restore the resting membrane potential and prepare the myocardial cells for the subsequent action potential, thereby maintaining the rhythm of the heart.

In summary, potassium's role is critical for repolarization during phase 3, making it the most crucial electrolyte at this stage of the cardiac action potential.